1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a pipe-shaped heating member to heat the fixing device effectively to shorten a warm-up time or a time to first print (hereinafter also “first print time”). Specifically, the heating member provided inside a loop formed by an endless belt member and facing the inner circumferential surface of the belt member is heated by an internal heater so as to heat the belt member. A stationary member is provided inside the loop formed by the belt member and presses against a rotary pressing member via the belt member to form a nip between the belt member and the rotary pressing member through which the recording medium bearing the toner image passes. The belt member and the rotary pressing member apply heat and pressure to the recording medium to fix the toner image on the recording medium.
The pipe-shaped heating member may be manufactured by bending a metal plate into a C-like shape in cross-section, so that an opening is extended in the long direction of the heating member perpendicular to the direction of rotation of the belt member. However, given such a shape, the inherent spring-back of the metal plate of the heating member, which attempts to return the heating member to its original shape after compression, may enlarge the opening in the heating member. Accordingly, the outer circumference of the heating member may be enlarged irregularly and may contact the inner circumferential surface of the belt member. Consequently, the belt member may be unevenly heated by the heating member, resulting in uneven fixing of the toner image on the recording medium.
To address this problem, the ends of the metal plate may be welded together to form an endless heating member without an opening. However, when the outer circumference of the heating member is substantially smaller than the inner circumference of the belt member, the heating member may not heat the belt member effectively. By contrast, when the outer circumference of the heating member is substantially greater than the inner circumference of the belt member, the heating member may not be installed inside the belt member easily.
Further, the belt member is tensioned by a greater force upstream from the nip in the direction of rotation of the belt member compared to downstream from the nip. Accordingly, the belt member may slacken downstream from the nip, enlarging the gap between the heating member and the belt member. Consequently, the heating member may not heat the belt member effectively. Moreover, the slacked belt member may disturb movement of the recording medium discharged from the nip, preventing smooth conveyance of the recording medium and creasing or wrinkling the recording medium.